Induction of hydroxycinnamoyl-tyramine conjugates in pepper by Xanthomonascampestris, a plant defense response activated by hrp gene-dependent and hrp gene-independent mechanisms

inoculation of pepper leaves, Capsicum annuum cv. Early Calwonder ECW 10R, with strains of Xanthomonas campestris led to an accumulation of the phenol ic conjugates feruloyltyramine (FT) and p-coumaroyltyramine (CT) 24 h posti noculation in nonhost- and gene-for-gene-determined incompatible interactio ns with X, campestris pv, campestris and X, campestris pv, vesicatoria, res pectively. In contrast, neither compound was detected in compatible interac tions with X, campestris pv, vesicatoria. The accumulation of FT and CT was preceded by an increase In the extractable activity of tyrosine decarboxyl ase as well as increases in the transcription of genes encoding phenylalani ne ammonia-lyase and tyramine hydroxycinnamoyl transferase. No such changes were detected in compatible interactions. Very rapid accumulation of FT an d CT occurred (4 h postinoculation) in pepper in response to a X, campestri s pv, campestris mutant carrying a deletion of the hrp gene cluster, In con trast, hrp mutants of X, campestris pv. vesicatoria failed to elicit the pr oduction of FT and CT. These observations suggest the existence of hrp gene -dependent and -independent activation mechanisms of a defense response inv olving hydroxycinnamoyltyramines.